The present invention relates to a method of adjusting a surface potential sensor for measuring the surface potential of a photoconductive element, or image carrier, included in an electrophotographic image forming apparatus.
Generally, a laser printer, electrophotographic copier, facsimile transceiver or similar electrophotographic image forming apparatus has an image carrier in the form of a photoconductive drum or a photoconductive belt. In this kind of apparatus, a charger uniformly charges the surface of the photoconductive drum or belt to a predetermined potential, and then an exposing device electrostatically forms a latent image on the charged surface by illuminating it imagewise. A developing device develops the latent image by a toner. The resulting toner image is transferred to a paper sheet fed from a paper feeding section included in the apparatus. A surface potential sensor may be located to face the photoconductive element for measuring the surface potential of the element, as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 95255/1981 and 83743/1988. In such a case, the voltages to be applied to, among others, the charger and developing roller are controlled to optimal values on the basis of the surface potential of the drum or belt measured by the surface potential sensor, thereby stabilizing the image quality. A volume for adjustment is usually associated with the surface potential sensor and operable to adjust the sensor in matching relation to the varying degree of deterioration thereof, i.e., the frequency of use. Such adjustment is extremely troublesome and usually performed by a serviceman. However, the surface potential sensor cannot be provided with high accuracy since the adjustment relies on manual work, i.e., differs from one serviceman to another. While a highly accurate and reliable surface potential sensor may be incorporated in the image forming apparatus, it increases the overall cost of the apparatus and, therefore, cannot be applied to inexpensive apparatuses.
In light of the above, a reference plate in the form of an electrode may be located to face the measuring surface of the surface potential sensor in order to adjust the sensor automatically, as also taught in Japanese Patent Laid-Open Publication No. 95255. Such an implementation has a problem that the reference plate or electrode increases the cost of the apparatus. Another problem is the potential which is not necessary for image formation and remains on the surface of the photoconductive element due to aging and changes in ambient conditions. Specifically, even if the surface potential of the photoconductive element is measured with accuracy, some special means has to be provided for cancelling the potential remaining on the photoconductive element, complicating the construction and control of the apparatus.
To reduce the cost of the apparatus, use may be made of a relatively inexpensive surface potential sensor which has distance dependency. However, such a sensor cannot be used unless the distance between the sensor and the reference plate or electrode is the same as or substantially same as the distance between the sensor and the photoconductive element, again resulting in a complicated construction.